1. Field of the Invention
Pressure-maintaining device The invention relates to a pressure-maintaining device for a fuel supply system of an internal combustion engine of a motor vehicle, having a pressure reservoir which is intended to be connected to a supply line between a fuel pump and the internal combustion engine, has a pressure chamber, has a pretensioned, movable wall in order to bound a volume of fuel in the pressure chamber and has means for registering the position of the movable wall, it being possible for the fuel pump to be regulated by means of a regulating device as a function of the position of the movable wall.
2. Related Art
It is well know to insert pressure regulators into fuel supply systems wherein the regulators regulate a fuel pump as a function of the position of the diaphragm of the pressure regulator (See, for example, U.S. Pat. No. 3,824,974; U.S. Pat. No. 1,837,504; U.S. Pat. No. 4,728,264). The disadvantage of these arrangements is that, in spite of the fuel pump being regulated, pressure fluctuations in the fuel supply system occur because of internal and external influences.
Such fuel supply systems are already used experimentally in motor vehicles and are therefore known. The pressure reservoir is used for the intermediate storage of fuel and permits cyclic operation of the fuel pump. The pressure reservoir has two limit switches which limit the reciprocating travel of the movable wall. As soon as the pressure chamber has been filled, the regulating device switches off the fuel pump. The internal combustion engine is then supplied with fuel by the pressure reservoir. After the pressure chamber has been virtually completely emptied, the fuel pump is switched on again and supplies the internal combustion engine with fuel. The pressure-maintaining device is used in particular instead of a return line, which is provided to return excess fuel from the internal combustion engine into a fuel tank. By comparison with a fuel supply system having a return line, the pressure-maintaining device has the advantage that the power demand of the fuel pump may be reduced up to 95%. This leads, in particular in city traffic, to a reduction in the fuel consumption of the motor vehicle.
A disadvantage in the case of the known pressure-maintaining device is that, while the fuel pump is operating, a higher pressure prevails in the fuel supply system than when the internal combustion engine is being supplied from the pressure reservoir with the fuel pump switched off. These pressure differences have their origin in the pretensioning forces, which are variable over the reciprocating travel of the movable wall, and in the influence of frictional forces on the movement of the movable wall. Pressure fluctuations in the fuel supply system are produced in this way, and have to be compensated for by a pressure-regulating valve. However, this pressure-regulating valve is a component which is susceptible to faults and very costly.